The present invention involves a differential pressure sensor. More particularly, the present invention deals with compensating a pressure sensor for changes in static pressure.
There are currently many applications for differential pressure sensors. Such sensors often include a sensor housing having an inner chamber. A deflectable diaphragm is inserted within the chamber dividing the chamber into two cavities. A first pressure is provided to the first cavity, while a second pressure is provided to the second cavity. Based on the difference in the first and second pressures, the diaphragm deflects.
The diaphragm also typically includes a conductive portion separated from, but aligned with conductive portions on the inner walls of the cavities. The conductive portion on the diaphragm forms a capacitive plate. Similarly, the conductive portions on the inner walls of the cavities also form capacitive plates. Therefore, the capacitive plate on the diaphragm forms a first variable capacitor with the capacitor plate on the interior wall of the first cavity, and a second variable capacitor with the capacitor plate on the interior wall of the second cavity. As the diaphragm deflects due to the differential pressure, the capacitive values of the two variable capacitors change. The pressure sensor provides an output signal representative of the ratio of capacitive values of the variable capacitors, and based on those capacitive values, the differential pressure is determined.
However, problems can arise due to nonlinearities in such a capacitive pressure sensor. For example, stray capacitances in the system can cause nonlinearities which must be compensated.
Also, errors can result due to changes in static pressure. Static pressure, also commonly referred to as line pressure, can be defined in several ways. For example, the first and second pressures provided to the first and second cavities of the pressure sensor may have values of, 2990 psi(P.sub.L) and 3000 psi(P.sub.H). In that case, the differential pressure is 10 psi (3000 psi-2990 psi). Static pressure is sometimes defined as the average of P.sub.H and P.sub.L, or 2995 psi. Static pressure may also be defined simply as P.sub.H, or P.sub.L alone. However, regardless of which definition is used for static pressure, errors in the output signal of the pressure sensor can result based on variations in static pressure.
It is desirable for the differential pressure sensor to provide an output signal which is unaffected by variations in static pressure. For example, where P.sub.H =3000 psi and P.sub.L =2990 psi, the differential pressure is 10 psi, and the static pressure is 2995 psi (using the average of P.sub.H and P.sub.L as the measurement for static pressure). However, where P.sub.H =10 psi and P.sub.L =0 psi, the static pressure is 5 psi, but the differential pressure is still 10 psi. Due to certain stresses placed on the housing of the pressure sensor, the output signal of a typical differential pressure sensor may vary 1% per 1000 psi variation in static pressure. Thus, with the example given above, the output signal from the differential pressure may vary significantly with the change in static pressure.
The Frick U.S. Pat. No. 4,370,890, discloses a mechanical configuration for a differential pressure sensor which attempts to eliminate, or compensate for, the unwanted mechanical stresses on the pressure sensor housing due to variation in static pressure. This helps to eliminate variations in the output signal of the differential pressure sensor due to variation in static pressure. However, there is a continuing need for compensation schemes which compensate for variations in the output signal due to variations in static pressure and which can be adjusted by electrical rather than mechanical means.